Paper webs made from cellulosic fibers are used in consumer products such as paper towels, toilet tissue, and facial tissue. Multi-ply paper structures are well known in the art. It is generally understood that a multi-ply structure can have an absorbent capacity greater than the sum of the absorbent capacities of the individual single plies which make up the multi-ply structure. Without wishing to be limited by theory, it is thought that this difference is due, at least in part, to the inter-ply storage space created by the addition of an extra ply.
Multi-ply paper products may have two or more plies positioned in face to face relationship and joined together. Each ply can be formed from a paper web. A paper web can have one or more layers as it is formed on a paper machine, as is also well known in the art.
Papermaking is generally understood to be a process in which paper is produced into large rolls, and wherein the large rolls are converted. Paper converting may be described as a processing step in which paper is used to fabricate another paper product. For example, paper converting may include operations such as embossing, slitting, sheeting, grooving, punching, and folding. Some converting operations may also include perforating, blocking, binding, gluing, and laminating as well.
For example, converting may include the embossing and marrying of two or more plies to form a multi-ply paper structure. The individual plies of a multi-ply paper structure may be joined in any number of suitable ways, including adhesive bonding or mechanical bonding, such as by embossing. Frequently, plies are embossed for aesthetic reasons, to provide space between adjacent plies, and to connect adjacent plies in face to face relationship.
Embossing is typically performed by one of three processes, knob-to-knob embossing, nested embossing, and/or rubber-to-steel embossing. Knob-to-knob embossing comprises axially parallel rolls juxtaposed to form a nip between the crests of the embossing knobs on opposing rolls. Nested embossing comprises axially parallel rolls juxtaposed to form a nip where the embossing knobs on one roll mesh between the embossing knobs of the other roll. Rubber-to-steel embossing comprises a steel roll with embossing knobs opposed to a roll having an elastomeric roll cover wherein the two rolls are axially parallel and juxtaposed to form a nip where the embossing knobs of the embossing roll mesh with the elastomeric roll cover of the opposing roll.
For example, during the knob-to-knob embossing process of a two-ply paper web, each paper web is fed through separate nips formed between separate embossing rolls and pressure rolls where embossing knobs on the embossing rolls produce compressed regions in each paper web. The two paper webs are then fed through a common nip formed between the embossing rolls where the embossing knobs on the two rolls bring the paper webs together in a face-to-face contacting relationship.
By comparison, nested embossing works by having the crests of the embossing knobs on one embossing roll intermesh with the embossing knobs on the opposing embossing roll at the nip formed between the two rolls. As a paper web is passed between the two embossing rolls, a pattern is produced on the surface of the paper web by the interconnectivity of the knobs of one roll with the open spaces of the opposing roll.
Comparatively, rubber-to-steel embossing works by having one hard embossing roll having embossing knobs in a desired pattern and a back-side soft impression roll, often having an elastomeric roll cover aligned in an axially parallel configuration to form a nip between the rolls. As a paper web is passed through the nip between the rolls, the embossing knobs impress the web against and into the rubber to deform the structure of the web.
It is possible to marry two or more paper webs together using adhesive. In an exemplary nested embossing process, an adhesive applicator roll may be aligned in an axially parallel arrangement with one of the two embossing rolls forming a nip therewith, such that the adhesive applicator roll is upstream of the nip formed between the two embossing rolls. The adhesive applicator roll transfers adhesive to the embossed paper web on the embossing roll at the crests of the embossing knobs. The crests of the embossing knobs typically do not touch the perimeter of the opposing roll at the nip formed therebetween necessitating the addition of a marrying roll to apply pressure for lamination. The marrying roll forms a nip with the same embossing roll forming the nip with the adhesive applicator roll, downstream of the nip formed between the two embossing rolls.
It is also known in the art to marry two or more paper webs autogenously (without adhesives) by high pressure lamination. With high pressure lamination, the adhesive applicator roll is eliminated and, in some embodiments, the marrying roll may be replaced with a steel anvil roll. In addition to bonding the paper webs, high pressure lamination produces a visually distinctive embossment pattern exhibiting a glassine appearance which is decoratively pleasing.
Despite the variety of embossing/marrying techniques that are known in the prior art, a common problem that occurs when producing an embossed, multi-ply paper product is the mis-registration that may occur during the converting processes. For example, in the nested embossing processes of the prior art, the transformations of embossing, adhesive application and marrying (laminating one ply to another ply) ideally occur in the same localized area. In a product where embossing, adhesive application, and/or marrying do not occur in the same localized area, the product may suffer from a relatively low ply-bond strength, less-pronounced embossments, or double embossments—a result of an emboss roll and marrying roll not striking the paper in the same area.
Without wishing to be limited by theory, it is thought that one possible cause of misregistration of plies in prior art processes/using prior art apparatus is the loss of tension in a paper web that may occur during embossing which may be magnified by the high interactive forces between the adhesive and the paper web. In this scenario, it may be possible for a paper web with a relatively low tension to “lift off” and go out of phase with the ply with which it is to be laminated because of interactions that the paper web may have with the adhesive applicator roll. As a result of the loss of tension/“lift off”, the paper web may lose tension and consequently may slip out of position, thus causing the resultant embossed multi-ply product to be misregistered.
Thus it is desirable to provide an apparatus and process for manufacturing a multi-ply paper product wherein the apparatus and process provides improved registration control such that a paper substrate having adhesive applied thereon is able to maintain registration of the aforementioned during an embossing process. The present invention overcomes the limitations of the prior art by providing increased stability to the paper substrates during the embossing and marrying transformations in order to reduce, and in some cases, eliminate misregistration between plies.